In fuel injection systems for use in internal combustion engines, it is known to provide one or more fuel pumps to supply pressurised fuel to the fuel injectors of the engine. The fuel injectors are mounted in a cylinder head assembly of the engine, alongside inlet and exhaust gas valves. The cylinder head assembly is mounted to a cylinder block of the engine, in which the engine cylinders are formed, so that the tips of the injectors and the inlet and exhaust valves are exposed to the respective combustion spaces defined by the cylinders.
The inlet and exhaust valves are operated by one or more main camshafts, which are mounted either in the cylinder head assembly in an overhead camshaft engine, or in or on the engine block in a cam-in-block engine. In typical cam-in-block engines, push-rods cooperate with associated cams on the camshaft to transfer drive from the camshaft to pivoting rocker arms in the cylinder head assembly, which in turn actuate the respective gas valves. In overhead camshaft engines, push rods are not required and instead the rocker arms can be driven directly by the cams.
A high-pressure fuel pump generally includes a plunger that is driven for reciprocal movement within a bore provided in the pump housing by means of a cam drive arrangement. Typically, the cam drive arrangement includes a cam and a cam follower (such as a roller). The cam follower cooperates directly or indirectly with an engine-driven cam, so as to drive a pumping stroke of the plunger during which fuel is pressurised. In a high-pressure medium- or heavy-duty engine, several fuel pumps are often used to supply the necessary pressure and volume of fuel. The components of each pump may be mounted in a modular pump housing.
The fuel pumps must be positioned in the engine in a location where a suitable engine-driven cam can be provided to drive the pump. Furthermore, to minimise the cost and complexity of the fuel injection system, it is desirable that the fuel pumps are mounted relatively close to the fuel injectors. Accordingly, several ways of arranging the fuel pumps have been developed.
In one known fuel injection system, each of the injectors has an integrated pump assembly, mounted in line with the electronically-controlled injector. Such injectors are typically known as unit injectors. In these systems, it is necessary to drive the fuel pumps along a direction coaxial with the cylinder head bores in which the fuel injectors are mounted. In an overhead camshaft engine, the pumps may be driven using a rocker-arm arrangement in which additional cams are mounted on the main camshaft. Alternatively, the pumps of the unit injectors can be driven by a push-rod arrangement in a cam-in-block engine.
FIG. 1 of the accompanying drawings illustrates a known arrangement of this type in an overhead camshaft engine. FIG. 1 is a view of parts of the cylinder head assembly, viewed from above, with the rocker cover removed. The camshaft 10, which is driven by a timing wheel 12 connected to the crankshaft 10 with a belt or chain (not shown), extends horizontally or longitudinally along the cylinder head assembly. The camshaft is held in place by a plurality of journal bearings, which are housed in bearing caps or housings 14. The bearing housings are mounted to a cylinder head block (not shown) by way of bolts 16.
The camshaft 10 carries a plurality of cams (not shown), each of which actuates an associated rocker arm 18a, 18b, 18c. The rocker arms are pivotable about a rocker arm shaft 20, which extends parallel to the camshaft 10. Three rocker arms are provided for each cylinder of the engine: an inlet valve rocker arm 18a actuates the inlet valves 22a of the cylinder; an outlet valve rocker arm 18b actuates the outlet valves 22b of the cylinder; and an injector rocker arm 18c actuates the pump mechanism of a unit injector 24 mounted vertically in the cylinder head block. Each group of three rocker arms 18a, 18b, 18c is separated from the group of three rocker arms associated with an adjacent cylinder by one of the bearing housings 14, so that the bearing housings 14 are equally spaced along the camshaft 10 and are aligned with the gaps between adjacent cylinders of the engine.
When unit injectors are used, fuel that is pressurised within each unit injector may be used exclusively in that injector, as is the case in the arrangement shown in FIG. 1. Alternatively, each unit injector may be connected to an accumulator rail or common rail, which serves as a reservoir for high-pressure fuel for all of the injectors. In such an arrangement, a combination of unit injectors (with integrated pumps) and non-pumping injectors (which receive high-pressure fuel only from the common rail) may be used to reduce the cost and complexity of the system.
FIG. 2 of the accompanying drawings, which is similar to FIG. 1 and in which like reference numerals are used for like parts, illustrates a common-rail system in which unit injectors with integral pump mechanisms 24 are used in combination with non-pumping injectors 26. Each of the injectors 24, 26 is connected by high-pressure fuel pipes 28 to a common accumulator rail 30, which in this example extends parallel to the camshaft 10. Advantageously, the cost and complexity of this system is reduced compared to the FIG. 1 arrangement, since fewer moving parts are required.
Advantageously, the arrangements shown in FIGS. 1 and 2 are relatively compact, and in particular the distance through which the high pressure fuel supplied by the pumps must be conveyed to reach the injectors is relatively short. Also, the fuel injection components can all be accommodated in the cylinder head assembly, and covered by the rocker cover of the engine to damp the noise from the pumps. However, the use of rocker arms to drive the pumping mechanisms of the pumps can result in undesirable side-loads on the pumping mechanism. Furthermore, the rocker arms that drive the pumps are usually heavy, to ensure durability and stiffness, and so the pumps must be equipped with a high-force return spring to ensure that the cam follower of each pump remains in contact with its associated rocker arm at all engine speeds.
Further disadvantages can arise from the integration of the pump components with the fuel injection components. In particular, in integrated unit injectors, the space available for the fuel injector components is restricted and the injector components are subjected to the undesirable cyclic loads that are generated in the pumping mechanism.
Another known system, which avoids some of the above-described problems with unit injector systems, employs linear-type unit pumps that are separate from the injectors. Each unit pump may be linked to a respective injector, or all of the unit pumps may be linked to a common fuel rail from which the injectors are supplied. As will be appreciated from FIGS. 1 and 2, due to space constraints in the cylinder head assembly, it is not generally possible to mount such stand-alone (i.e. non-injecting) unit pumps in the cylinder head.
Another known system is known from U.S. Pat. No. 6,354,278 disclosing an engine for an outboard motor wherein a fuel pump is mounted laterally to the engine block.
Accordingly, the unit pumps are typically mounted within or on the engine block, and are driven from a drive shaft mounted in or on the block. Unit pump systems are therefore best suited for cam-in-block engines, in which the unit pumps can be driven from the block-mounted gas valve camshaft. If it is desired to use a unit pump arrangement in an overhead camshaft engine, it is necessary to provide a dedicated fuel pump driveshaft in or on the block, adding considerably to the cost and complexity of the system. Furthermore, because the pumps are mounted in or on the engine block, noise and vibration from the pumps can be transmitted directly into the engine bay, and the high-pressure fuel pipes that convey the fuel from the pumps to the injectors need to be relatively long and routed externally to the engine.
Another arrangement utilises a remote fuel pump, such as a radial fuel pump, which is mounted remotely from the injectors and is connected to a common rail by a suitable high-pressure fuel line. The remote pump is typically mounted outside the engine block, and is driven by a drive gear arrangement linked to the engine crankshaft. In such systems, it is necessary to add an auxiliary drive wheel to the engine to drive the gear arrangement for the pump, and the remote pump itself must include its own camshaft, bearings, cam housing, and lubrication system for the drive mechanism.
Against this background, it would be desirable to provide an arrangement of fuel pumps for a fuel injection system of an internal combustion engine in which the cost and complexity of the system is minimised and in which the above-mentioned problems with known arrangements are reduced or mitigated.